Physics 0625 | Paper 6 Mock 01 | Alternative to Practical

(a) The ammeter in Fig. 8.1 shows the same current for every length tested:

current = 0.40 A [1]

(b) Using \( R = \dfrac{V}{I} \) for each row:

For example, at 20 cm: \( R = \dfrac{0.52}{0.40} = 1.3 \; \Omega \). [2]

(c) and (d) The graph of \( R \) against \( l \):

Axes correctly labelled with quantities and units. [1] All five points plotted correctly. [2] A straight line of best fit passes through or near all points and through the origin. [1]

(e) Resistance is directly proportional to length. As the length increases, the resistance increases in the same ratio. The graph is a straight line through the origin, confirming \( R \propto l \). [1]

This is because a longer wire presents more metal atoms for the drifting electrons to collide with, increasing the opposition to current flow.

(f) Reading from the graph at \( l = 50 \) cm:

\( R \approx 3.25 \; \Omega \) (accept 3.0 to 3.5 from a correctly drawn graph). [1]

This is found by going up from 50 cm on the x-axis to the line of best fit, then reading across to the R-axis.

(g) Two improvements to length measurement accuracy:

1. Ensure the wire is taut and straight along the rule, not sagging or kinked, so the measured length matches the actual wire length between the clips. [1]

2. Measure the length from the inside edge of each crocodile clip, as only the wire between the clips carries the test current. Alternatively, use a rule with mm markings for more precise readings, or repeat each length measurement and take the mean. [1]

(a) From the protractor scale on Fig. 5.1, the incident ray makes an angle of approximately 40° with the normal N. [1]

The angle of incidence is read between the incoming ray and the dashed normal line using the protractor markings on the semicircular scale.

(b) Dividing \(\sin i\) by \(\sin r\) for each row:

[1 for correct division method; 1 for at least five values correct to 2 d.p.]

Each ratio is found by dividing the sin i value by the corresponding sin r value. For example, at \(i = 10°\): \(\frac{0.174}{0.122} = 1.43\).

(c) The mean value:

\[ \text{mean} = \frac{1.43 + 1.52 + 1.46 + 1.47 + 1.49 + 1.47}{6} \; [1] \]

\[ = \frac{8.84}{6} = 1.47 \; [1] \]

(d) The mean value of \(\frac{\sin i}{\sin r}\) represents the refractive index of the perspex. [1]

By Snell's law, \(n = \frac{\sin i}{\sin r}\), so this ratio is constant for a given material and equals its refractive index.

(e) As the light enters the perspex from air, it bends towards the normal. [1]

Light passing from a less dense medium (air, \(n \approx 1\)) into a denser medium (perspex, \(n \approx 1.5\)) slows down. The reduction in speed causes the wavefront to change direction, bending the ray towards the normal at the point of incidence. [1]

(f) One source of random error: difficulty reading the protractor to the exact degree, due to the finite thickness of the ray and the precision of the protractor scale. [1]

Other valid sources include parallax when reading angles, slight variation in positioning the ray at the intended angle, or the width of the light beam making the exact centre of the ray ambiguous.

(g) The student should repeat each measurement at least three times at each angle of incidence, calculate the mean value of \(r\) for each \(i\), and then recalculate \(\sin i / \sin r\). Averaging multiple readings reduces the impact of random scatter. [1]

(h) The independent variable is the angle of incidence. [1]

This is the variable the student deliberately changes between trials. The angle of refraction (the dependent variable) is the measured outcome that responds to the change.

(a) The gold leaf rises (deflects away from the central rod). [1]

The charged rod induces a redistribution of charge in the electroscope: electrons in the cap are repelled down the rod and onto the leaf, giving both the rod and the leaf the same sign of charge, so they repel.

(b)

[1] for polythene correct. [1] for both glass and copper correct.

Polythene gains a negative charge and glass gains a positive charge when rubbed, both of which cause the leaf to rise. Copper is a conductor, so any charge produced immediately flows away through the student's hand, and the leaf shows no response.

(c) When polythene is rubbed with wool, friction transfers electrons from the wool onto the polythene rod. [1] The polythene gains extra electrons and therefore acquires an overall negative charge. [1]

(d) Any one valid improvement: repeat each test three times and check for consistency / ensure the rod is fully discharged (e.g. by touching it to a metal surface) before each test / carry out the experiment on a dry day or in a dry environment to reduce charge leakage through moisture. [1]

(e) A glass rod rubbed with silk acquires a positive charge. [1]

Electrons are transferred from the glass to the silk during rubbing, leaving the glass with a deficit of electrons and hence a net positive charge.

(a) When \( R_v \) is at maximum, the ammeter reads:

\( I = 0.05 \text{ A} \) [1]

(b) Total resistance when \( R_v \) is at maximum:

[1]

This total includes both the variable resistor and the lamp in series.

(c) Resistance of the lamp at \( I = 0.05 \text{ A} \):

[1]

(d) Resistance of the lamp at \( I = 0.50 \text{ A} \):

[1]

(e) The lamp resistance is higher at the greater current (11.6 Ω vs 10 Ω) because the larger current heats the filament to a higher temperature. In metals, increased temperature causes lattice ions to vibrate more vigorously, increasing collisions with conduction electrons and raising the resistance. [1]

(f) Power dissipated in the lamp at each setting using \( P = IV \):

At \( R_v \) = maximum:

[1]

At \( R_v = 0 \):

[1]

The lamp is far brighter at the lower resistance setting because it dissipates over 100 times more power.

(g) The voltage across the lamp ranges from 0.5 V to 5.8 V. [1]

The variable resistor cannot produce exactly 0 V across the lamp (some current always flows), and the terminal p.d. cannot reach the full 6.0 V because of the cell's internal resistance.

(h) A variable resistor provides continuous, fine control over the current and voltage in the circuit without needing multiple power supplies or component swaps. A single adjustment sweeps through the full range smoothly. [1]

(i) Plan for plotting the full V-I characteristic: [2]

1. Starting with \( R_v \) at maximum, record the ammeter reading \( I \) and the voltmeter reading \( V_L \). [1]
2. Gradually decrease \( R_v \) in small steps, recording \( I \) and \( V_L \) at each step until \( R_v \) reaches zero. Plot \( V_L \) on the x-axis against \( I \) on the y-axis and draw a smooth curve of best fit through the points. [1]